Why blood types differ?

Question

Why blood types differ?

Answer 1

The blood types are due to antigens on the red blood cells. There are two alleles, A, and B. When both are absent, the blood type is O.

Answer 2

DUE TO ANTIGENS PRESENT  IN RBC

Answer 3

The ABO blood type is controlled by a single gene with three (not 2) alleles: i, IA, and IB. These alleles  determine the antigens on the surface of red blood cells.

IA allele gives type A, IB gives type B, and i gives type O. IA and IB are dominant over i, so ii people have type O; IAIA or IAi have A; and IBIB or IBi have type B. IAIB people have both phenotypes because A and B express a special dominance relationship: codominance, which means that type A and B parents can have an AB child